Inchworm linear motors are compact piezoelectromechanical actuators and known for use in apparatus such as linear actuators and positioners.
Disclosed in U.S. Pat. No. 3,902,084, and incorporated by reference herein, is piezoelectric electromechanical translation apparatus, the load activating shaft of which extends through a housing and is programmably movable over long distances with extremely fine resolution, in extremely small incremental steps by a piezoelectric driver which has three driver sections in end-to-end relationship around the shaft and which is referenced to the housing. The driver is operative to clamp the shaft, and when a staircase voltage is applied to an element thereof, translates the shaft in a direction and over an incremental distance related to the polarity and amplitude of the steps of the staircase voltage. Staircase voltage cycles may be repeated to move the shaft incrementally over a long distance.
There is also disclosed, in U.S. Pat. No. 3,902,085, which is also incorporated by reference herein, electromechanical translation apparatus which provides translation with a high degree of uniformity of motion. The device includes a piezoelectric driver having three driver sections arranged in end-to-end relationship around the shaft. This driver is referenced to a housing and provides forces for moving the shaft with respect to the housing. The sections of the driver are interconnected by bridging members which assemble the driver sections in integral relationship and yet allow movement of the driver sections into and out of engagement with the shaft without imparting undersired motion to the shaft.
Operation of the apparatus disclosed in the two above-referenced U.S. Patents comprises a sequence of steps which involve applying voltages to the driver sections which, for purposes of clarity, will be referred to as forward, center and rear sections with respect to the desired direction of movement of a shaft about which the driver is arranged.
In operation, a voltage is applied to the forward section thus causing it to clamp the shaft. Then a variable rate staircase voltage is applied to the center section causing it to expand and, as the driver is mounted fixed in relation to its housing, the forward section is pushed forward by the extent of the expansion of the center section and, therefore, moves the shaft in a forward direction along a corresponding distance.
A voltage is then applied to the rear section causing it to clamp the shaft and the voltage on the forward section is subsequently removed, the grip of the forward section on the shaft, therefore, being loosened. The staircase voltage, its upper limit having been reached prior to the voltage being applied to the rear section, then starts downward towards its lower limit, causing the center section to contract to its original length. After the center section has contracted, a voltage is then applied to the forward section causing it to clamp the shaft, and the sequence of steps outlined above are repeated.
A disadvantage of the apparatus described above is that in the sequence of steps which constitute its operation, there are steps which involve the simultaneous clamping of the shaft by both the forward and rear sections of the driver.
Systems, in which the translation apparatus or inchworm motors are as described above, resist sudden changes in velocity, such as are caused by clamping the shaft with the rear and forward sections. Simultaneous clamping, therefore, by the front and rear sections generates undesirable stresses on the system as a whole and in particular, causes undesirable wear of the driver sections and of the shaft.
Furthermore, the need for a `wait` stage, in which both sections are clamped, provides for a relatively slow rate of travel of the shaft.